Elementary Steps and Dynamic Aspects of Carrier-Mediated Cation Transport Through Membranes: the Streptogramin Antibiotics (Group B)

Summary

Certain streptogramin antibiotics of the B group strongly interact with membranes, a fact that was hitherto unknown. According to electrical measurements on phospholipid bilayer membranes the peptide-lactone virginiamycin S facilitates the transport of protons, alkali ions, and alkaline earth ions across these membranes, thus acting as a general cation mediator. The cation specificity is H⁺ >> Rb⁺ > K⁺ > Cs⁺ > Na⁺ ≈ Ca²⁺. The pH-dependent interaction between phospholipid membranes (dispersions) and the antibiotic has been investigated employing spectroscopic methods. Virginiamycin S is bound predominantly to the surface region of the membrane; the dissociation constant of the membrane-bound compound is 7 × 10^-6 M for the monodeprotonated neutral state and 3.7 × 10^-5 M for the corresponding zwitterionic state whereas it is only 1 × 10^-4 M for the negatively charged, twice deprotonated state of the antibiotic. About 250 phospholipid molecules form a binding site for one antibiotic molecule. The affinity to the membrane is caused mainly by interaction of the cyclic peptide-lactone ring of the antibiotic with the membrane.

The elementary steps as well as the dynamic aspects that are relevant to the mechanism of action of the proton and metal ion transport mediated by virginiamycin S have been investigated employing spectroscopic techniques and chemical relaxation methods. The kinetics of the proton transfer reactions of virginiamycin S have been studied in 33.8% water in methanol (vol/vol) and required a quantitative differentiation between the various protolytic states of the 3-hydroxypicolinyl residue of the antibiotic, which acts as proton acceptor and donor. In this medium its protolytic properties are similar to those shown when it is membrane-bound. At about pH 7, the overall proton transfer occurring at the 3-hydroxypicolinyl residue of the antibiotic is due mainly to an intramolecular process; the rate constant for proton transfer from the ring nitrogen to the phenolic oxygen is 850 sec^-1 and that for the reverse process is 670 sec^-1. Evidence was found which suggested that slow conformational rearrangements of the peptide residue of virginiamycin S take place with rate constants of about 10² sec^-1. In nonpolar solvents and those of intermediate polarity, virginiamycin S forms stable complexes with cations. Complex formation between alkaline earth ions, and to a certain extent also between alkali ions and the antibiotic, is attributed to the twice and three times deprotonated protolytic states of the 3-hydroxypicolinyl residue; this is consistent with similar studies on simple model compounds.